47
pools and to warn against pollution. Both play an important
role for health and the environment.
The knowledge of the water constituents allows the prediction of fishing grounds and thereby providing economic
benefits and sustainable exploitation of the oceans. In the
case of natural disasters, satellite imagery provides a quick
analysis of the extent and the impact finding quick ways for
evacuation and first aid.
For example, the people in Cape Town, South Africa,
suffer from severe drought since 2017 ongoing until now
and the fresh water supply is strongly restricted since the
beginning of 2018 due to a decrease of the largest reservoir,
the Theewaterskloof Dam, to around 13% of its average
capacity (A. Voiland, 2018-01-30, https://earthobservatory.
nasa.gov/IOTD/view.php?id=91649, accessed 09 February
2018).
Environmental changes are observed by the variation in
the constituents, the water extent, or the water level. Satellite
images show erosion changes along coastlines or the growth
of islands. Tracking phytoplankton supports fisheries, the
transportation industry, and tourism industry identifying
regions of high fish content (Moreira and Pires 2016), which
they can either systematically avoid or locate. In order to
warn against the toxicity of the HABs, governmental institutes are interested in identifying and tracking phytoplankton
using remote sensing (Schaeffer et al. 2015).
The water availability and the water cycle play an important role in climate change. Climate models benefit from an
improved understanding of changes and mechanisms analyzed
from data retrieved from satellite remote sensing. Additionally,
performing photosynthesis, phytoplankton is part of the carbon cycle and consume carbon dioxide, which is a major contributing agent in the frame of global warming. Field et al.
(1998) reported that around 47% of the total net primary production is performed in marine ecosystems. Thus, it is of high
importance to retrieve and understand the variability of the
water constituents on all available temporal and spatial scales
which already can almost be covered by remote sensing.
Overview of Technical Details
Platforms
The sensors that measure signals from the Earth’s atmosphere, waters and land surfaces are mounted on satellite
platforms. Each satellite flies in a specific orbit around the
Earth and is loaded with power supplies, navigation tools,
and support systems for the instruments. Generally, the most
common satellite orbits are geostationary or polar-orbiting,
which leads to differences in spatio-temporal resolutions.
Geostationary satellites continuously monitor specified geographical locations above the Earth’s surface in height of
approximately 36,000 km. Therefore, they cannot cover the
complete globe. For example, the Geostationary Ocean
Color Imager GOCI onboard the Communication Ocean and
Meteorological Satellite 1 (COMS) captures images over
Korean waters eight times a day (Ryu et al. 2012). Usually,
television and communication satellites operate in this orbit
due to the stable location.
Polar-orbiting satellites circle around the globe in approximately 100 min at a height of about 700–800 km. Their sensors
are capable to cover the entire surface of the earth. The time to
receive a full coverage depends on the sensor’s swath (the scanning line or area on the ground) and can last from 2 to several
days. The sensor Moderate Resolution Imaging Spectrometer
(MODIS) onboard the platforms Aqua and Terra has a revisiting
time of less than 3 days due to its large swath of 2330 km (Xiong
et al. 2005). Polar-orbiting satellites are usually sun-synchronous: They cross the equator at the same local time (LT). Aqua
passes the equator from South to North (ascending node) at
1:30 p.m. LT and Terra has an equator-crossing time of
10:30 a.m. LT in a descending node (Xiong et al. 2005).
Figure 5a illustrates the product chlorophyll a calculated from
MODIS/Terra for all available orbits for 2017-07-28.
Instruments
The measurement sensors or instruments are installed
onboard the platform. There are two main measurement
techniques. MODIS is a whiskbroom scanner, which oscillates across the satellite flight direction. Subsequently, it
scans a part of the swath area from one side to the other and
backwards while the satellite continues moving (Xiong et al.
2005). A sensor with a pushbroom measuring technique does
not rotate: The whole swath width is scanned at once and
pushed forward with the satellite flight direction and movement. The Medium Resolution Imaging Spectrometer
MERIS onboard the Environmental Satellite (Envisat) is a
prominent example (ESA 2006).
Most of the remote sensing instruments in space have
multiple measuring bands or channels to detect a certain
spectral range of light and its intensity. Mainly, a channel is
defined by its central wavelength and the band width
described by an individual response function. The function
determines the ability of a band to detect a specific part of the
electro-magnetic spectrum. For instance, the MODIS band 1
ranges from 620–670 nm detecting all photons within this
wavelength range (Xiong et al. 2005). The response function
defines how much of an infinitesimal wavelength interval
contributes to the finally measured signal at this band.
Chlorophyll a exhibits interesting features with an increase
in absorption towards 670 nm in this spectral range (Bricaud
et al. 1998). Using measuring bands with a large band width
leads to a loss of specific spectral features which particularly
reduces the information quality in water bodies. Therefore, it
is important to carefully specify the spectral settings of a
channel depending on the sensor’s objective.
There are spectrometers with a higher spectral resolution
than MERIS or MODIS. For example, the latest space-borne
Marine Optics and Ocean Color Remote Sensing
pools and to warn against pollution. Both play an important
role for health and the environment.
The knowledge of the water constituents allows the prediction of fishing grounds and thereby providing economic
benefits and sustainable exploitation of the oceans. In the
case of natural disasters, satellite imagery provides a quick
analysis of the extent and the impact finding quick ways for
evacuation and first aid.
For example, the people in Cape Town, South Africa,
suffer from severe drought since 2017 ongoing until now
and the fresh water supply is strongly restricted since the
beginning of 2018 due to a decrease of the largest reservoir,
the Theewaterskloof Dam, to around 13% of its average
capacity (A. Voiland, 2018-01-30, https://earthobservatory.
nasa.gov/IOTD/view.php?id=91649, accessed 09 February
2018).
Environmental changes are observed by the variation in
the constituents, the water extent, or the water level. Satellite
images show erosion changes along coastlines or the growth
of islands. Tracking phytoplankton supports fisheries, the
transportation industry, and tourism industry identifying
regions of high fish content (Moreira and Pires 2016), which
they can either systematically avoid or locate. In order to
warn against the toxicity of the HABs, governmental institutes are interested in identifying and tracking phytoplankton
using remote sensing (Schaeffer et al. 2015).
The water availability and the water cycle play an important role in climate change. Climate models benefit from an
improved understanding of changes and mechanisms analyzed
from data retrieved from satellite remote sensing. Additionally,
performing photosynthesis, phytoplankton is part of the carbon cycle and consume carbon dioxide, which is a major contributing agent in the frame of global warming. Field et al.
(1998) reported that around 47% of the total net primary production is performed in marine ecosystems. Thus, it is of high
importance to retrieve and understand the variability of the
water constituents on all available temporal and spatial scales
which already can almost be covered by remote sensing.
Overview of Technical Details
Platforms
The sensors that measure signals from the Earth’s atmosphere, waters and land surfaces are mounted on satellite
platforms. Each satellite flies in a specific orbit around the
Earth and is loaded with power supplies, navigation tools,
and support systems for the instruments. Generally, the most
common satellite orbits are geostationary or polar-orbiting,
which leads to differences in spatio-temporal resolutions.
Geostationary satellites continuously monitor specified geographical locations above the Earth’s surface in height of
approximately 36,000 km. Therefore, they cannot cover the
complete globe. For example, the Geostationary Ocean
Color Imager GOCI onboard the Communication Ocean and
Meteorological Satellite 1 (COMS) captures images over
Korean waters eight times a day (Ryu et al. 2012). Usually,
television and communication satellites operate in this orbit
due to the stable location.
Polar-orbiting satellites circle around the globe in approximately 100 min at a height of about 700–800 km. Their sensors
are capable to cover the entire surface of the earth. The time to
receive a full coverage depends on the sensor’s swath (the scanning line or area on the ground) and can last from 2 to several
days. The sensor Moderate Resolution Imaging Spectrometer
(MODIS) onboard the platforms Aqua and Terra has a revisiting
time of less than 3 days due to its large swath of 2330 km (Xiong
et al. 2005). Polar-orbiting satellites are usually sun-synchronous: They cross the equator at the same local time (LT). Aqua
passes the equator from South to North (ascending node) at
1:30 p.m. LT and Terra has an equator-crossing time of
10:30 a.m. LT in a descending node (Xiong et al. 2005).
Figure 5a illustrates the product chlorophyll a calculated from
MODIS/Terra for all available orbits for 2017-07-28.
Instruments
The measurement sensors or instruments are installed
onboard the platform. There are two main measurement
techniques. MODIS is a whiskbroom scanner, which oscillates across the satellite flight direction. Subsequently, it
scans a part of the swath area from one side to the other and
backwards while the satellite continues moving (Xiong et al.
2005). A sensor with a pushbroom measuring technique does
not rotate: The whole swath width is scanned at once and
pushed forward with the satellite flight direction and movement. The Medium Resolution Imaging Spectrometer
MERIS onboard the Environmental Satellite (Envisat) is a
prominent example (ESA 2006).
Most of the remote sensing instruments in space have
multiple measuring bands or channels to detect a certain
spectral range of light and its intensity. Mainly, a channel is
defined by its central wavelength and the band width
described by an individual response function. The function
determines the ability of a band to detect a specific part of the
electro-magnetic spectrum. For instance, the MODIS band 1
ranges from 620–670 nm detecting all photons within this
wavelength range (Xiong et al. 2005). The response function
defines how much of an infinitesimal wavelength interval
contributes to the finally measured signal at this band.
Chlorophyll a exhibits interesting features with an increase
in absorption towards 670 nm in this spectral range (Bricaud
et al. 1998). Using measuring bands with a large band width
leads to a loss of specific spectral features which particularly
reduces the information quality in water bodies. Therefore, it
is important to carefully specify the spectral settings of a
channel depending on the sensor’s objective.
There are spectrometers with a higher spectral resolution
than MERIS or MODIS. For example, the latest space-borne
Marine Optics and Ocean Color Remote Sensing